The present invention broadly relates to a method for fabricating a rim gear and, more specifically, pertains to a new and improved method for fabricating in particular, but not exclusively, a starter rim gear. The present invention further relates to a rim gear produced in accordance with this method.
Generally speaking, the method of the present invention for fabricating rim gears, particularly starter rim gears and the like from sheet metal, such as sheet steel or steel plating, comprises the step of providing a dished blank having a substantially cylindrical rim portion extending substantially parallel to the axis of rotation of the dished blank at one side thereof and the step of cold forming an outer surface of the substantially cylindrical rim portion according to the well known Grob method to effect on the outer surface an external gear tooth system or toothed profile having proper gear wheel or gear characteristics thereon, thereby also effecting an internal gear tooth system or toothed profile on the inner surface of the substantially cylindrical rim portion.
In order to avoid any misunderstandings from the outset, the well known Grob method of cold forming or cold working will be here briefly elucidated, even though this method is extensively known in the cold forming art from documentation as well as from practice:
This method simultaneously generates both an outside profile and an inside profile, which can be different from one another. The workpiece, in this application the tubular or substantially cylindrical rim part, is mounted onto a mandrel or plug corresponding to the inner profile. The workpiece then has imparted thereto a workpiece feed by axially moving it and rotating it about its lengthwise axis. During this time the workpiece is externally worked by annularly or ring-like profiled forming rolls or rolling tools, whereby each forming roll or rolling tool performs or carries out single or individual impacting forming operations or blows in rapid succession or sequence, which operations are synchronized with or accommodated to the pitch of the profile or toothing and the feed of the workpiece. These single impacting forming operations or blows are accomplished or carried out in the same sense of direction and predominantly in the lengthwise direction of the profile. The individual or single impacting forming operations or blows consecutively carried out by the same forming roll or rolling tool lie in a helical or screw-like zone. This helical or screw-like zone is determined by the feed of the workpiece. Seen in the lengthwise direction of the profile or toothing, the individual, consecutive impacting forming operations or blows in the same tooth gap partially overlap each other. During each single impacting forming operation or blow, material along a relatively small section of the workpiece is pushed into one of the depressions of the mandrel, mainly flowing in a radial direction. A method of this kind pertaining or directed to relatively thick-walled workpieces is disclosed in Swiss Pat. No. 579,427, in French Pat. No. 7,538,539 and in German Patent Publication No. 2,549,230. Also significant to the Grob method or process is the disclosure of German Pat. No. 1,016,222, published Sept. 26, 1957, the disclosure of which is incorporated herein by reference.
The designation "starter" stands in most cases for a device which sets, for example, combustion engines in motion.
Motor vehicles and, in particular, automobiles or cars are generally equipped with electric gear wheel cranking starters. This kind of starter works in conjunction with a car battery powering an electric motor and drives a pinion. During the starting operation, this pinion is meshed with a starter gear wheel attached to the engine or flywheel of the engine.
It is therefore necessary that starter gear wheels possess proper gear or gear wheel characteristics.
In contrast to other sheet metal gear transmissions used in motor vehicles, such as toothed belt pulleys, clutch discs and splined tubing, the gear tooth system of the rim gear wheel according to this invention features real gear wheel geometry or gear wheel characteristics, that is to say, real gear wheels or gears in the sense that they have relatively high tooth heights (radially measured) and small tooth flank widths (axially measured) for precise meshing with another gear such as required for starter gear wheels or gears which are intended to mesh with another gear, namely the associated pinion.
These special requirements often cause difficulties during manufacture of sheet metal starter gear wheels according to the Grob method.